Servomechanisms are generally implemented across a broad spectrum of industries. Such devices typically make use of a sensor to produce output in response to movement of a rotor shaft and a microprocessor to convert the sensor output into a useful metric. In particular, the sensor featured in such applications may include a potentiometer, an optical encoder, and/or a resolver.
Electric motors that require controlled armature current waveforms (in order to rotate smoothly, for example) also require accurate rotor position sensing. Some motors use sensorless technologies, but these technologies generally do not provide accurate rotor position sensing at low speeds and are not smooth on startup of the motor. Other motors are not adapted to utilize sensorless technologies and must incorporate a rotor position sensing mechanism. Conventional motors typically use either an encoder or a resolver in conjunction with associated electronic circuitry to determine rotor positions. Depending on the resolution required, these solutions may be prohibitively expensive within applications that require low cost motors.
Many electric motor applications generally require smooth rotation and/or accurate control. Brushless motors typically achieve this by using 3-phase, sine-wave commutation and accurate rotor position detectors, usually in the form of an encoder or a resolver. The accurate rotor position detector generally operates to ensure that the sine wave remains synchronized with the rotor, thus avoiding commutation-induced torque ripple. Conventional methods used for accurately detecting rotor positions usually employ encoders and resolvers.
Encoders sense mechanical motion and translate the sensed motion into electrical signals, of which optical encoders are the most common type. An optical encoder typically includes a housing to support precision bearings and electronics, a shaft with a sensing disc having alternating clear and opaque segments, a light emitting diode (LED), and a photo transistor receiver. A beam of light produced by the LED is aimed at the optical disc. When the optical disc rotates, the light beam passes through the clear segments but is blocked by the opaque segments so that the optical disc effectively pulses the light beam. The pulsed light beam is received by the photo transistor receiver. The photo transistor receiver and the circuitry inside the encoder together provide signal to a motor controller outside the encoder and can also perform functions such as improving signal/noise. Encoders in their simplest form have one output to determine shaft rotational speed or to measure a number of shaft revolutions. Other encoders have two outputs and can provide direction-of-rotation information as well as speed and number of revolutions. Still other encoders provide an index pulse which indicates absolute rotor position. The description thus far relates specifically to incremental encoders, which upon startup, the absolute position of the encoder is not known.
A second type of encoder, called an absolute encoder, has a unique value for each mechanical position throughout a rotation. These units typically consist of the incremental encoder described above with the addition of another signal channel that serves to generate absolute position information, typically with diminished accuracy. For an absolute encoder that is provided with an index pulse, the accuracy improves once the rotor traverses the index pulse. Incremental encoders may be acceptable within asynchronous motors, where speed feedback is important. Absolute encoders may be desirable in synchronous motor applications, where both position and speed feedback are important.
Another class of high resolution encoders is that of sine/cosine encoders, which generate sine and cosine signals rather than pulse waveforms. When used with additional electronic components, processor capability and software, sine/cosine encoders indicate rotor position with fine resolution.
In general terms, encoders of all types are precision built, sensitive devices that must be mechanically, electrically and optically matched and calibrated. Resolvers, on the other hand, typically provide one signal period per revolution and are known to he highly tolerant of vibration and temperature variation. A typical use of this technology may include a resolver generating two signals, both a sine-wave signal and a cosine-wave signal, for each revolution. An advantage of using resolvers is that they provide absolute rotor position information, rather than incremental information as is typically the case with most encoders. One disadvantage of resolvers is that they operate on the principle of inductive coupling of magnetic fields which are modulated and subsequently demodulated into sine and cosine components. The processes involved with coupling the magnetic fields and modulation and demodulation limit the resolver's useful speed range. As with encoders, resolvers are precision built, commercially available sensing devices that may be fragile and expensive, requiring complex encoding/decoding circuitry and bulky and heavy magnetic components.
Ring magnets and digital Hall effect sensors are often used as rotor position sensing mechanisms within brushless direct current (DC) motor applications where square-wave or six-step drive is used. This method of sensing provides low resolution, typically six position steps per electrical cycle when using three sensors. Six-step drive generally does not require high resolution rotor position sensing. At the same time, these drive methods do not generally provide ripple-free torque from the motor. This may be unacceptable in a variety of applications.
U.S. Pat. No. 6,522,130 is drawn to a device for sensing rotor position and detecting rotational speed over a broad range of electric motor speeds with analog Hall effect sensors. That notwithstanding, the structural design requires the addition of a sensor ring, and does not describe the process for estimating rotor position. Thus, the approach described therein does not lend itself to scalable applications in terms of device dimensions, placement of sensors, accuracy of angular measureiment—and notably—the discrete hardware implementation of obtaining angular position data.
While useful in certain applications, conventional servomechanism sensing and control devices have a number of disadvantages, including, for example, the inherent volume exclusion of the devices themselves, ease of manufacture, cost and accuracy of angular position measurements. These characteristics tend to limit the design flexibility and commercial feasibility of conventional sensor systems.